Because of this listing, a temperature TMDL is being developed. EPA was asked by state and tribal governments to take the lead in coordinating development of temperature TMDLs for the Columbia and Snake Rivers. The main reasons for this are that there are federally owned hydroelectric projects on the Columbia, and there are inter-state and tribal issues involved.

To understand how EPA is developing the temperature TMDL, you need to think about the length of the Columbia from the Canadian border to the ocean.

The key concept is that the water from Canada to the ocean naturally warms up. This is particularly true in the summer when it's hot. So for the water downstream to be cooler, Lake Roosevelt's water needs to be the coolest of the cool. Technically, that's why Ecology has set a temperature criteria of 16º Celsius (61ºF) in Lake Roosevelt. In the lower Columbia, it's 20º Celsius (68ºF).

The graphic shows the temperature criteria created for three reaches of the Columbia. This criteria uses "natural conditions" as a reference point. These conditions reflect environmental conditions before human changes. The criteria sets an allowable amount that human activities can warm the river above natural conditions.